Flow control system and method

ABSTRACT

A flow control system include a congestion detecting section and control section. The congestion detecting section detects congestion in a packet switching network. The control section is arranged in a transmitting node. When the congestion detecting section detects congestion, the control section calculates a new transmission packet rate. When the new transmission packet rate is smaller than a current transmission packet rate, the control section changes the current transmission packet rate to the new transmission packet rate after transmission of data to be transmitted to a receiving node. A flow control method is also disclosed.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a flow control system and method for avoiding any congestion at a node connected to a packet switching network to execute communication protocol processing.

[0002] Currently, packet switching networks (IP networks) using the Internet protocol (IP) are widely used. In an IP network, end-to-end congestion is avoided between transmitting and receiving nodes. That is, in protocol layer 4 serving as the transport layer of the reference model of OSI (Open Systems Interconnection), congestion is avoided by flow control of TCP (Transmission Control Protocol). TCP is a connection-oriented transfer protocol which transfers data on the basis of a virtual circuit (VC). In the flow control of TCP, a window control scheme is used. When a transmitting node detects packet loss in a network, the cause for it is determined as congestion, and the transmission data amount (congestion window) is reduced to ½. A congestion window is an estimated value of the transfer capability of a network.

[0003] For more efficient congestion control, ECN (Explicit Congestion Notification) which explicitly notifies a network of a symptom of congestion has been proposed. In ECN, each packet has congestion indication information. A relay node in a network catches a symptom of congestion (when the transmission buffer occupation amount of the relay node becomes large) and sets the information. A receiving node returns to a transmitting node a packet having such congestion information. With this scheme, the transmitting node reduces the congestion window to ½, as in the case of packet loss.

[0004] The operation of TCP is described in Japanese Patent Laid-Open No. 2000-134279 (reference 1). References to be mentioned include “TCP Timeout and Retransmission”, TCP Illustrated Vol. 1 (Addison Wesley, 1994), Chapter 21, pp. 297-322 and Internet Engineering Task Force (IETF), Request for Comments (RFC): 2001/2481, January 1997/January 1999 (references 2 and 3). Reference 1 discloses a method of avoiding unnecessary congestion window reduction when packet loss/rejection has occurred due to not congestion but a line error. Japanese Patent Laid-Open No. 2000-115239 (reference 4) discloses a means for controlling packet retransmission for communication which is managed for each packet and has low management priority while monitoring the congestion state.

[0005] An IP network is originally used as a network for data transmission. However, it is recently used for real-time transmission (streaming) of stream data such as voice or image data. The easiest streaming is transmission of stream data encoded at a fixed bit rate. As a transmission protocol suitable for streaming, the Internet Engineering Task Force (IETF), Request for Comments (RFC): 1889, January 1996 (reference 5) is known. However, RTP has no congestion avoiding function. For this reason, if data in an amount beyond the network capacity is to be transmitted, congestion occurs, and packet loss often happens. While congestion is continuing, voice is interrupted, and an image becomes inaccurate. In addition, data transfer using TCP cannot be executed. The reason for this is as follows. Upon detecting congestion, TCP halves the congestion window. However, if congestion continues due to RTP, data that can be transmitted by TCP is exponentially decreased (repeatedly halved) and becomes almost zero.

[0006] On the other hand, when TCP is used as a transmission protocol, the transmission bit rate changes in accordance with the state of the network. Hence, the time required until the end of transmission is undefined. For file transfer, only the wait time until transfer is ended becomes long. This does not affect the quality (when all the contents are eventually transferred to the receiving side, no problem occurs in the quality). However, in streaming, reconstruction is sometimes interrupted, resulting in a large degradation in quality (i.e., a time factor is important). That is, in streaming, not only avoiding congestion is necessary but also transmission must be executed while maintaining an expected time continuity (otherwise, reconstruction is stopped halfway, resulting in adverse influence on the quality).

[0007] A method of avoiding halfway stop of reconstruction as much as possible while avoiding congestion in streaming has been proposed. In this scheme, a plurality of stream data with different encoding bit rates are prepared and selectively transmitted in accordance with the state of the network. This scheme will be referred to as a stream switching scheme hereinafter. Generally, stream data cannot be simply switched halfway. For this reason, in the stream switching scheme, points (switching points) at which stream data can be switched are prepared for each stream data at a predetermined synchronous time interval (e.g., a 1-sec interval).

[0008] For example, Japanese Patent Laid-Open No. 2000-83029 (reference 6) discloses an example of a VOD (video On Demand) system which is a kind of stream switching scheme using an ATM (Asynchronous Transfer Mode) network. In this system, resources are ensured and congestion information is acquired using the function of the ATM network, stream data to be transmitted is switched at a switching point, and the transmission packet rate is updated. However, an IP network itself does not have this function. Since end-to-end congestion is avoided, the technique disclosed in reference 6 cannot be directly applied to the IP network.

[0009] In realizing the stream switching scheme on an IP network, the transmission bit rate of TCP is estimated, and stream data is selected and transmitted in accordance with the transmission bit rate. However, in the window control scheme, since a congestion window is controlled, the transmission bit rate is not directly defined. In addition, the delay time in the network, which changes for each packet, directly leads to a variation in transmission bit rate. For this reason, an application measures the amount of data that could be actually transmitted, calculates an average transmission bit rate that is average for a given time, and uses the average transmission bit rate as an estimated value of the transmission bit rate after that.

[0010] However, since switching points are present only at a predetermined time interval, continuous reconstruction is not always guaranteed. For example, assume that the average transmission bit rate of the TCP is 1 Mbps, and stream data of 1 Mbps (1,000 kbps) is transmitted in accordance with the transmission bit rate. Assume that the average transmission bit rate suddenly changes to 100 kbps. If stream data (500 kbits) corresponding to 0.5 sec remains until the next switching point, the 500-kbit data is sent at 100 kbps (5 sec is required for transmission). For this reason, an extra delay of 4.5 sec occurs, and reconstruction stops for 4.5 sec.

[0011] This problem that reconstruction stops halfway is posed even when an encoding means capable of encoding data in real time is used as an application. Generally, an encoding means has an internal buffer (for, e.g., 0.5 sec at maximum) to smooth the output bit rate. Hence, a code sequence that is already generated and is present in the buffer must be output at the bit rate at the time of encoding. For example, when the transmission bit rate of TCP is 200 kbps, and a 100-kbit encoded code sequence is present in the buffer, the contents of the buffer must be output within 0.5 sec. However, if the transmission bit rate of TCP suddenly changes to 100 kbps, the time required for output changes to 1 sec. When viewed from the receiving node, the delay suddenly increases by 0.5 sec. For this reason, reconstruction stops for 0.5 sec.

[0012] As a measure against this problem, a reconstruction delay buffer for a long time (e.g., 10 sec or more) is prepared in the receiving node. If the delay due to the variation in transmission bit rate falls within this range, halfway reconstruction stop can be avoided, and the probability of continuous reconstruction can be increased. Hence, a large delay buffer is generally used in streaming using TCP as a transmission protocol. However, although the probability of continuous reconstruction can be increased, halfway reconstruction stop cannot be completely eliminated. It is difficult to execute a dialogue-type application such as a video phone.

[0013] As described above, in the prior arts, if congestion is avoided, reconstruction is stopped halfway. Additionally, to increase the probability of continuous reconstruction, a large reconstruction delay buffer must be inserted to the receiving side. For this reason, high-quality streaming must be abandoned, or the band for streaming must be ensured in advance at the time of network design (a design that eliminates any congestion is necessary).

SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to provide a flow control system and method which simultaneously achieve congestion avoidance and continuous reconstruction.

[0015] In order to achieve the above object, according to the present invention, there is provided a flow control system for avoiding congestion in a packet switching network, comprising congestion detection means for detecting congestion in the packet switching network, and control means, arranged in a transmitting node, for, when the congestion detection means detects congestion, calculating a new transmission packet rate, and when the new transmission packet rate is smaller than a current transmission packet rate, changing the current transmission packet rate to the new reduced transmission packet rate after transmission of data to be transmitted to a receiving node.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a block diagram of a stream data transmission system according to the first embodiment of the present invention;

[0017]FIG. 2 is a block diagram showing an example of an application 11 shown in FIG. 1;

[0018]FIG. 3 is a block diagram showing another example of the application 11 shown in FIG. 1; and

[0019]FIG. 4 is a block diagram of a stream data transmission system according to the second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The present invention will be described below in detail with reference to the accompanying drawings.

[0021]FIG. 1 shows a stream data transmission system according to the first embodiment of the present invention. Referring to FIG. 1, a transmitting node 1 and receiving node 2 are connected through a communication network 5 formed from a plurality of relay nodes 3 and 4. Stream data is transmitted from the transmitting node 1 to the receiving node 2.

[0022] The transmitting node 1 comprises an application 11 which generates stream data to be transmitted, a transmitting section 12 which forms packets of the stream data generated by the application 11 at a transmission packet rate designated by a control section 14 and transmits the data packets, a congestion detecting section 13 which detects congestion which has occurred in the communication network 5 that connects the transmitting node 1 and receiving node 2, the control section 14 which controls the transmission packet rate of the transmitting section 12 in accordance with a result from the congestion detecting section 13, and a packet processing section 15 which executes input/output processing with respect to the communication network 5.

[0023] The receiving node 2 comprises a packet processing section 15 which executes input/output processing with respect to the communication network 5, a receiving section 22 which receives reception packets from the packet processing section 15, and an application 21 which uses the received stream.

[0024] The application 11 in the transmitting node 1 supplies a data sequence to the transmitting section 12 in accordance with a transmission timing supplied from the transmitting section 12. Upon receiving a notification about a new reduced transmission packet rate from the control section 14, the application 11 designates a wait time necessary for transition to the control section 14. The transmitting section 12 generates a transmission timing in accordance with the transmission packet rate designated by the control section 14 and notifies the application 11 of it. The transmitting section 12 assigns a series of sequence numbers to the data sequence supplied from the application 11 to form packets and supplies them to the packet processing section 15. If no data sequence is supplied from the application 11 at the transmission timing, a free packet is generated and supplied to the packet processing section 15. The wait time necessary for transition means a time that is necessary for the application 11 to change the transmission packet rate. The internal arrangement of the application and the mechanism for calculating the wait time necessary for transition will be described later.

[0025] The application 21 in the receiving node 2 consumes (uses) stream data supplied from the receiving section 22. The receiving section 22 receives packets from the packet processing section 15, reconstructs the data sequence, and supplies it to the application 21. In addition, to notify the transmitting node 1 of the reception situation, the receiving section 22 generates an Ack packet containing information about the total number of reception packets with the series of sequence numbers and the sequence number of the packet that was finally received in the reception packets with the series of sequence numbers, and supplies the Ack packet to the packet processing section 15.

[0026] The packet processing sections 15 send, to the communication network 5, packets received from the transmitting section 12 and receiving section 22 and supply packets received from the communication network 5 to the congestion detecting section 13 and receiving section 22. The above-described Ack packet is transferred from the receiving node 2 to the transmitting node 1 through the packet processing sections 15. The congestion detecting section 13 detects congestion on the basis of the contents described in the Ack packet and notifies the control section 14 of the congestion. When congestion, i.e., packet loss occurs somewhere in the communication network 5, a shift corresponding to the packet loss is generated in the relationship between the final sequence number and the total number of reception packets. The congestion detecting section 13 monitors the difference between the final sequence number and the total number of reception packets described in the Ack packet and detects congestion from a change in this difference.

[0027] When the congestion detecting section 13 detects no congestion, the control section 14 increases the transmission packet rate at a predetermined acceleration and designates it to the transmitting section 12 as a new transmission packet rate. Simultaneously, the application 11 is also notified of the new transmission packet rate. The notification to the application 11 may be made after a lapse of a predetermined wait time (or after transmission of data to be transmitted).

[0028] When the congestion detecting section 13 detects congestion, the control section 14 obtains a new reduced transmission packet rate by multiplying the transmission packet rate by a predetermined coefficient smaller than 1. Next, the control section 14 notifies the application 11 of the new transmission packet rate. After notification, when a predetermined wait time designated to the application 11 has elapsed, the control section 14 designates the new transmission packet rate to the transmitting section 12.

[0029] The wait time is designated from the application 11 to the control section 14 by designating the maximum value of transition time necessary for the application 11 as a wait time at the time of activating the application. Alternatively, every time a notification of a new transmission packet rate is received, a transition time necessary at that time may be designated as a wait time. The wait time may be represented not by the time itself but by the amount of data to be transmitted before deceleration of the transmission packet rate. In this case, the control section 14 converts the data amount into time as a wait time.

[0030] In the first embodiment, after the elapse of the wait time (time necessary for transition of the application) necessary for the application, the transmission packet rate is changed to a lower rate. With this arrangement, any delay due to a sudden change in bit rate, i.e., stop of reconstruction can be avoided. According to this embodiment, congestion avoidance and quality maintenance (preventing any sudden delay viewed from the receiving side) can be simultaneously achieved.

[0031]FIG. 2 shows an example of the application 11. The application 11 is constituted by an encoding section 111 which encodes an input signal, a buffer 112 which temporarily stores the output from the encoding section 111, and an application control section 113 which controls the encoding section 111. The encoding section 111 encodes an input signal at a quantization step size designated by the application control section 113. Stream data generated by encoding is supplied to the buffer 112.

[0032] The buffer 112 buffers the stream data from the encoding section 111. The buffer 112 extracts a data sequence having a predetermined length at a transmission timing designated by the transmitting section 12 and supplies the data sequence to the transmitting section 12. The application control section 113 monitors the occupation amount (stored data amount) of the buffer 112, controls the quantization step size in accordance with a transmission packet rate supplied from the control section 14, and designates the quantization step size to the encoding section 111. Upon receiving a notification of a new reduced transmission packet rate from the control section 14, the application control section 113 calculates the necessary transition time by dividing the occupation amount of the buffer 112 by the current transmission packet rate. The calculated value is designated to the control section 14 as a wait time.

[0033]FIG. 3 shows another example of the application 11. The application 11 is constituted by storage devices 114 and 115 which store encoded data, a read section 116 which reads out the encoded data stored in the storage devices 114 and 115, a buffer 117 which buffers the data output from the read section 116, and an application control section 118 which controls the read section 116.

[0034] The storage device 114 stores stream data encoded at a low bit rate as a file. The read section 116 reads out the file from the storage device 114 and supplies the low-bit-rate stream data to the buffer 117. The storage device 115 stores stream data encoded at a high bit rate as a file. The read section 116 reads out the file from the storage device 115 and supplies the high-bit-rate stream data to the buffer 117. These encoded stream data have switching points at, e.g., a 1-sec interval (any predetermined fixed time interval can be used). The storage devices 114 and 115 store stream data encoded at two kinds of, i.e., high and low bit rates. However, the number of kinds of bit rates is not limited to two. Data of three or more kinds of bit rates may be stored.

[0035] The read section 116 reads out stream data from one of the storage devices 114 and 115 in accordance with a designation from the application control section 118 at a speed corresponding to the bit rate of the encoded stream data and supplies the stream data to the buffer 117. In addition, a wait time (time necessary for transition of the application) until the next switching point is supplied to the application control section 118. The buffer 117 buffers the stream data supplied from the read section 116 and reads out a data sequence having a predetermined length at the transmission timing designated by the transmitting section 12. The readout data sequence is supplied to the transmitting section 12.

[0036] The application control section 118 selects one of the storage devices 114 and 115, which stores stream data encoded at a bit rate corresponding to the transmission packet rate supplied from the control section 14. The application control section 118 designates the read section 116 to read out the stream data stored in the selected storage device at a speed corresponding to the encoding bit rate. Upon receiving a notification of a new transmission packet rate from the control section 14, the application control section 118 designates to the control section 14 as a wait time a time until the next switching point obtained from the read section 116.

[0037] In some cases, the encoding bit rate of selected stream data is equal to or lower than the bit rate determined on the basis of the transmission packet rate, and no data sequence is present in the buffer 117 at the transmission timing designated by the transmitting section 12. In such a case, the transmitting section 12 supplies a free packet to the packet processing section 15.

[0038] The second embodiment of the present invention will be described next with reference to FIG. 4. In this embodiment, instead of designating a wait time, an application designates a control section to approve a decrease in transmission packet rate. That is, an application 11 a sends a designation to a control section 14 a when the decrease in transmission packet rate becomes possible. For this reason, the operations of the application 11 a and control section 14 are different from those in the first embodiment.

[0039] The application 11 a supplies a data sequence to a transmitting section 12 in accordance with a transmission timing supplied from the transmitting section 12. Upon receiving a notification of a new reduced transmission packet rate from the control section 14 a, the application 11 a designates the control section 14 a to approve deceleration after transmission of data to be transmitted before deceleration of the transmission packet rate is ended. That is, after preparation of the application 11 a is ended, an approval is sent to the control section 14 a.

[0040] When a congestion detecting section 13 detects no congestion, the control section 14 a increases the transmission packet rate at a predetermined acceleration and designates it to the transmitting section 12 as a new transmission packet rate. The application 11 a is also notified of the new transmission packet rate. When the congestion detecting section 13 detects congestion, the control section 14 a obtains a new reduced transmission packet rate by multiplying the transmission packet rate by a predetermined coefficient smaller than 1. Next, the control section 14 a notifies the application 11 a of the new transmission packet rate and waits until an approval is received from the application 11 a. After that, the control section 14 a designates the new transmission packet rate to the transmitting section 12.

[0041] In this embodiment, the same effect as in the first embodiment can be obtained except the operations of the application 11 a and control section 14 a.

[0042] For congestion detection by the congestion detecting section 13, explicit congestion information can be used. That is, each packet has congestion indication information (a congestion state can be set) like ECN, and the transmitting section 12 clears the congestion indication information and transmits it. The relay nodes 3 and 4 in the communication network 5 set the congestion indication information in accordance with the congestion state of the output path. The receiving section 22 obtains the total number of sets by counting packets having the congestion indication information set in reception packets. The congestion state is supplied to the transmitting node 1 by an Ack packet containing information about the total number of sets. In the transmitting node 1, the congestion detecting section 13 detects congestion from a change in total number of sets extracted from the Ack packet.

[0043] The present invention is effectively used in an IP network, as described above. However, the present invention can also be applied to any other packet switching networks. The present invention is especially effective for a problem such as image stop in real-time transmission (streaming) of image or voice stream data. However, the present invention can also be applied to any other data.

[0044] As has been described above, according to the present invention, the transmission packet rate is controlled not only to avoid congestion in the network. Instead, the transition time necessary for an application in decelerating the transmission packet rate is always taken into consideration. With this arrangement, operation can be executed while preventing any sudden increase in delay visible to the user. As a result, a flow control system which realizes continuous reconstruction while avoiding congestion can be provided. 

What is claimed is:
 1. A flow control system for avoiding congestion in a packet switching network, comprising: congestion detection means for detecting congestion in the packet switching network; and control means, arranged in a transmitting node, for, when said congestion detection means detects congestion, calculating a new transmission packet rate, and, when the new transmission packet rate is smaller than a current transmission packet rate, changing the current transmission packet rate to the new transmission packet rate after transmission of data to be transmitted to a receiving node.
 2. A system according to claim 1, wherein the packet switching network is a packet switching network using an Internet protocol.
 3. A flow control system for avoiding congestion in a packet switching network, comprising: congestion detection means for detecting congestion in the packet switching network; control means for calculating a new transmission packet rate in accordance with a congestion state detected by said congestion detection means; an application which generates data to be transmitted, in accordance with the new transmission packet rate calculated by said control means; transmission means for forming a packet of the data to be transmitted, which is generated by said application, at the new transmission packet rate calculated by said control means, thereby generating a data packet; and packet processing means for transmitting packet data supplied from said transmission means from a transmitting node to a receiving node through the packet switching network and supplying, to said congestion detection means, a data packet transmitted from the receiving node through the packet switching network, wherein when said congestion detection means detects congestion, said control means calculates the new transmission packet rate and notifies said application of the calculated new transmission packet rate, and after transmission of the data to be transmitted before changing the transmission packet rate is ended, designates the new transmission packet rate to said transmission means.
 4. A system according to claim 3, wherein when said congestion detection means detects no congestion, said control means calculates the new transmission packet rate by increasing the transmission packet rate, and when said congestion detection means detects congestion, said control means calculates the new transmission packet rate by decreasing the transmission packet rate.
 5. A system according to claim 4, wherein said control means designates the new decreased transmission packet rate to said transmission packet rate to said transmission means, and notifies said application of the new decreased transmission packet rate after transmission of the data to be transmitted.
 6. A system according to claim 4, wherein said control means designates the new increased transmission packet rate to said transmission means, and simultaneously notifies said application of the new increased transmission packet rate, and
 7. A system according to claim 4, wherein said control means designates the new increased transmission packet rate to said transmission packet rate to said transmission means, and notifies said application of the new increased transmission packet rate after transmission of the data to be transmitted.
 8. A system according to claim 3, wherein when the new transmission packet rate is calculated, said control means notifies said application of the calculated new transmission packet rate, upon receiving the new transmission packet rate supplied from said control means, said application designates as a wait time to said control means a value obtained by dividing an amount of the data to be transmitted before changing the transmission packet rate by a current transmission packet rate, and said control means designates the new transmission packet rate to said transmission means after the wait time designated by said application has elapsed.
 9. A system according to claim 3, wherein when the new transmission packet rate is calculated, said control means notifies said application of the calculated new transmission packet rate, and simultaneously, receives an approval of the new rate from said application and then designates the new transmission packet rate to said transmission means, upon receiving the new transmission packet rate supplied from said control means, said application sends the approval of the new rate to said control means after transmission of the data to be transmitted before changing the transmission packet rate is ended.
 10. A system according to claim 3, wherein said application comprises encoding means for encoding an input signal, buffer means for buffering an output from said encoding means, and application control means for controlling said encoding means, and when the new transmission packet rate is supplied from said control means, said application control means designates as a wait time to said control means a transition time calculated by dividing an amount of data stored in said buffer means by a current transmission packet rate.
 11. A system according to claim 3, wherein said application comprises a plurality of storage means for storing encoded data with different bit rates, read means for reading out the encoded data from said storage means, buffer means for buffering the encoded data from said read means, and application control means for controlling said read means, said plurality of storage means store data encoded at the different bit rates, the encoded data having switching points at a predetermined time interval, when the new transmission packet rate is supplied from said control means, said application control means selects, from said plurality of storage means, storage means which stores data encoded at a bit rate corresponding to the transmission packet rate supplied from said control means and executes control to read out the data at a rate corresponding to the encoding bit rate, and designates a time until a next switching point to said control means as a wait time, and said buffer means buffers the data read out by said read means and supplies the buffered data to said transmission means in accordance with a designation.
 12. A system according to claim 3, wherein said application designates a maximum value of a transition time necessary to change the transmission packet rate to said control means in advance as a wait time at a time of activating said application, and when the new transmission packet rate is calculated, said control means notifies said application of the new transmission packet rate and designates the new transmission packet rate to said transmission means after an elapse of the wait time.
 13. A system according to claim 3, wherein said transmission means assigns a series of sequence numbers to the packets to be transmitted, the receiving node generates a reply packet containing information of the total number of reception packets assigned the series of sequence numbers and a sequence number of a final reception packet in the reception packets assigned the series of sequence numbers, and transfers the reply packet to the transmitting node, and said congestion detection means detects congestion on the basis of a change in difference between the total number of reception packets and the sequence number of the final reception packet.
 14. A system according to claim 3, wherein each packet to be transmitted has congestion indication information capable of setting a congestion state when a relay node of the packet switching network is in the congestion state, said transmission means clears the congestion indication information and transmits each packet, the receiving node generates a reply packet containing information of the total number of packets having the congestion indication information set and transfers the reply packet to the transmitting node, and said congestion detection means detects congestion on the basis of a change in total number of packets having the congestion indication information set.
 15. A system according to claim 3, wherein when the data to be transmitted is not generated by said application, said transmission means transmits a free packet to said packet processing means.
 16. A system according to claim 3, wherein the packet switching network is a packet switching network using an Internet protocol.
 17. A flow control method of avoiding congestion in a packet switching network, comprising the steps of: detecting congestion in the packet switching network; calculating a new transmission packet rate in accordance with a detected congestion state; generating data to be transmitted, in accordance with the calculated new transmission packet rate; forming a packet of the generated data to be transmitted at the calculated new transmission packet rate, thereby generating a data packet; and transmitting generated packet data to a receiving node through the packet switching network, wherein the calculation step comprises the step of, when congestion is detected, calculating the new transmission packet rate, and the generation step comprises the step of changing the transmission packet rate to the new transmission packet rate after transmission of the data to be transmitted before changing the transmission packet rate is ended.
 18. A method according to claim 17, wherein the changing step comprises the step of, when a time corresponding to a value obtained by dividing an amount of the data to be transmitted before changing the transmission packet rate by a current transmission packet rate has elapsed, recognizing that transmission of the data to be transmitted before changing the transmission packet rate is ended. 